syscall(): Make abort message visible

When syscall() was asked to perform an unknown syscall, we printed a
message with debug() and then called abort(). Since, recently, the message
from debug() is not visible on the console.

So pass the message directly to abort(), to do the right thing with it.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>